1. Field of the Invention
This invention relates to methods and apparatus used in the design and manufacture of surfboards, sailboards or similar aquatic boards, referred to generically herein as “board” or “boards.”
2. Description of the Related Art
Surfboards and sailboards are similar in shape and basic structure—the board typically has a high strength exterior skin covering that protects and is supported by very low-density material in the interior core; in construction, moldable plastic is used for the compound curvatures and sharp trailing edge contours conducive to a low-drag hydrodynamic shape; the board's primary strength is usually derived from a woven fabric, made from high-strength glass, carbon or aramid fiber, that is imbedded in the plastic to form a fiber-reinforced plastic or plastic composite skin.
The composite skin, which is very thin, can be reinforced with specially manufactured high-density PVC sheet foam, end-grain balsa or honeycomb core materials to form a “structural sandwich” or “cored composite.” The stiff, lightweight core material, used as a substrate to separate the high-strength composite layers on either side, creates a fundamentally different structure—the sandwiched core delivers the stiffness and rigidity of much thicker material, but at a fraction of the weight, and provides the impact resistance and compressive strength the very thin layers of plastic composite lack.
The composite core materials and reinforcing fabrics impart a high degree of stiffness and very high strength but, unlike the weaker plastics and foamed plastics with which they are combined, have a limited capacity to conform to compound curvature (i.e., a surface that curves in two directions at once). Where the curvature is severe, divisions are necessary to prevent structural defects such as wrinkles in the reinforcing fabric, or the breakage and/or failure of the core material to conform to the required shape. Since a break in the continuity of either material causes a large reduction in strength, the placement of a division—usually referred to as a joint or seam—is critical to the overall structural integrity of the board.
Currently, with molded methods of production, or in custom “one-off” manufacture (i.e., when the board is fabricated by hand), a joint or seam is required to accommodate the sharp curvature at the board's perimeter edge or “rail”—this division creates a number of seemingly unrelated but very serious problems, which increase manufacturing costs and seriously compromise the board's potential strength.
In prior art molded manufacture, for example, the sharp curvature around the board's perimeter edge compounds a number of very basic drawbacks inherent in the mold's concave configuration itself—the structural problems and high manufacturing costs that result make the concave female mold of the prior art fundamentally unsuited for board production. The problems begin with the mold's inward curving surface: when the reinforcing fabric is saturated by hand, the resin naturally tends to flow out of the fiber and pools in the concave cavity of the mold; the mold's sharp edge contours then create a dam that makes it very difficult for the squeegee to completely remove the excess—the result is a weak, heavy resin-rich skin. In areas of severe concave curvature, wrinkles in the reinforcing fabric easily occur, and are difficult or impossible to remove—pulling the fabric taut tends to lift it from the surface of the mold; pushing on the fabric is analogous to pushing on a string, and causes wrinkles to (re)appear.
To minimize the above problems, in the prior art the mold is divided into top and bottom halves; with the relatively flat and shallow surface the fabric is easily aligned and much of the excess resin can be successfully removed—the placement of the part-line, however, is in the worst position possible: at the board's exposed perimeter edge. Because the division of the mold also breaks the continuity of the high-strength fiber, the mold-seam on the finished board has only a fraction of the strength of material where the fiber is fully intact. The design of the joint is then compromised by the limitations of the mold's concave surface. The mold-seam is far stronger when reinforcement is applied to the interior of the joint—the inside surface of the joint, however, becomes completely inaccessible once the mold is closed. The mold-seam is therefore typically reinforced after the board is removed from the mold; this adds weight to the already resin-rich skin, and sufficient rework to negate much of the labor-saving advantage.
The difficulty molding the board's interior foam core then raises production costs further still. Because the expansion of plastic foam involves heat (e.g., polyurethane foams undergo an exothermic reaction; steam is required to expand EPS “bead” foams), there is both an expansion and a very slight cooling contraction cycle in the molding of the foam—the slight cooling contraction makes it very difficult to pre-mold the board's interior foam core to sufficiently tight tolerances to eliminate potential voids between it and the interior surface of the closed mold and, when the expansion of the foam occurs in the mold, the cooling contraction begins before the foam has fully hardened, which often causes poor adhesion or an inconsistent skin-to-interior core bond.
To reduce the problem, in the prior art the foam is contained in an extremely strong mold and the very high outward pressure generated by the foam's expansion is used to compress the foam against the interior surface of the mold to enhance adhesion and attain an adequate skin-to-interior core bond. Drawbacks include the high cost of the mold (the mold typically has steel reinforcing jigs attached and is held in a hydraulic press or by other mechanical means to prevent buckling, separating or failure under the high pressure of the expansion) and, because of the compression of the foam against the surface of the mold, the higher density of the foam and added weight,.
The additional problem is that the plastic composite is thin and bendable, and the resin generally shrinks between five and six percent as it cures. The direction of shrinkage is primarily into the fiber and against the surface of the mold, where it is held in place by the perfect vacuum that develops as the resin hardens and cures. Because the two halves must eventually meet at a precise point around the perimeter, the mold functions to stabilize the laminate, and prevents distortion or shrinkage of the resin from creating a mismatch between the board's two opposing sides. The skin must then be fully cured and receive the support of additional material (ordinarily provided by the bond between the two opposing sides and the board's interior core) before it can be removed from the mold. The order of application is a major problem: the fact that the least stable and longest curing material (i.e., the composite skin) is applied to the mold first, and quickly curing foam(s) or pre-molded interior core structures are added later, lengthens the mold-cycle and causes very slow production.
a. Molded Methods of Production
With excess weight, high-capital costs, and lack of any competitive advantage in terms of price, the molded fiberglass skin/polyurethane foam core surfboards manufactured in the early nineteen-sixties, soon after the introduction of polyurethane foam, were derisively referred to as “pop-outs” due to their structural inferiority. The commercial production of molded hollow boards was attempted in the early nineteen-seventies, but was also very brief-absent the interior foam core, the lack of an effective joint between the board's top and bottom sides (see, e.g., U.S. Pat. No. 3,514,798 to Ellis) caused the mold-seam at the perimeter to split open with relatively modest impact; with higher impact often detaching the skin from the interior support structure, the damage was difficult or impossible to repair.
Reviewing prior art clearly shows the structural defects and compromises caused by the concave configuration of prior art female molds. U.S. Pat. No. 3,802,010 to Smith, for example, suggests that the mold-seam at the board's perimeter can be eliminated by dividing a conventional female mold into right and left halves, and laying the saturated fiberglass fabric into the mold in a single sheet. According to the invention, the centerline division means that there are no joints along either side or rail where the board is subject to the greatest beating during use.
What is completely ignored is the fact that the board's outline around the perimeter is roughly twenty percent longer than the straight line along the axis of symmetry—if the part-line is placed at the shortest distance between the nose and tail of the board, the fiberglass must elongate a total of ten percent per side to cover the perimeter of the mold, while maintaining its original length at the center. Since fiberglass is not elastic, the fabric must be carefully cut and trimmed to conform to the shape of the mold, or the ten percent that is excess will appear as folds and wrinkles in direct proportion to the differential in length.
The mold's deep internal cavity and lack of access makes it impossible to accurately trim and create an overlapping joint in the fabric at the perimeter of the mold, however, and also prevents the defects from being properly repaired. The sharp folds in the reinforcing fabric create voids if subsequent layers are applied on top—this precludes the possibility of adding fiberglass layers or the use of any composite core material at all, or using these materials to create a bonding/reinforcing flange between the two opposing sides. The two sides are therefore joined by pouring a very thick layer of adhesive into a concave depression in the foam core, creating a very weak and heavy mold-seam between the opposing sides. The invention suggests trading the well-known structural problems caused by the relatively shallow concave cavity of the prior art female mold, for the much larger defects of a very deep one.
The closely related U.S. Pat. No. 4,383,955 to Rubio et al. specifically identifies a number of the more obvious problems outlined above, and teaches a conventional solution: to improve access, the right- and left-hand mold configuration to Smith is given an extra division that turns it into quarters—with the four relatively flat mold surfaces, the fiberglass fabric can be successfully applied to the mold without wrinkles; moreover, with the accessible mold surface ordinary steps such as polishing, prepping and applying release agents to the mold become possible so that the board can subsequently be removed. From a structural or fabrication standpoint, however, there is no improvement at all—the extra division adds a mold-seam at the perimeter of the board, and neither disclosure addresses any of the well-known problems involved in molding the board's interior foam core.
In both inventions, the liquid pre-foam is poured into the mold cavity and allowed to rise parallel to the board's width. The foaming reaction of the polyurethane resin is deceptively simple, however, because when complete, even the mixing cup appears to make a perfectly acceptable mold. The hidden problem is that the expansion of the foam occurs before the resin begins to harden—because the mold configuration causes considerable upward movement during expansion, the foam's cellular structure tends to be destroyed against the interior surface of the board/mold, and released blowing agent or gas is concentrated in the same area area; this leaves large voids directly beneath the surface of the fiberglass skin, and little or no skin-to-interior core bond.
In the disclosure to Rubio, et al. the voids are identified, but the inventors incorrectly attribute the “soft spots” (i.e., the voids beneath the skin) to the expansion and contraction cycle of the foam (the soft spots are described as areas where the foam has “pulled away” from the fiberglass skin). They therefore suggest using a baffle to contain the expansion of the foam to compress it against the interior surface of the skin—a partial step towards the prior art method of containing the foam in a high-strength mold and hydraulic press. Neither invention has seen production, since prior art problems of molding the board's interior foam core, the length of the mold-cycle, and the weak or inadequate joint between the board's two opposing sides are neither noted nor addressed. In known methods of sailboard production, the latter two problems are “solved” by either foaming the resin matrix or eliminating the reinforcing fiber in the skin; both solutions therefore entail a major reduction in strength.
In low-cost methods of sailboard production, for example, blow-molding or rotationally molding techniques are used to blow or melt a thermoplastic resin to the surface of the mold; although this produces a continuous one-piece skin, production is relegated to beginner and entry-level sailboards due to the excess weight/inadequate strength caused by the lack of any composite material at all. The added drawback is that the interior foam core must be formed by injecting liquid pre-foam into the interior cavity of the closed mold, which involves the production problems outlined above.
Pre-molding the board's interior foam core is a very widely used alternative since it allows a major reduction in weight. The problem, as previously noted, is the difficulty of consistently pre-molding the foam to sufficiently tight tolerances to eliminate potential voids between the interior foam core and the surface of the mold. In the prior art, the lack of close tolerances is compensated for by saturating the reinforcing fabric with an epoxy resin that has a blowing agent added; with the laminate/interior core assembly contained in a heated mold and a mechanical press, the epoxy expands to fill any voids or discrepancies between the pre-molded interior foam core and the closed cavity of the mold. With the added stiffness of a PVC sheet foam layer sandwiched between layers of foamed laminate in the skin, the board is light in weight, and the quickly-curing foamed resin and rapid mold-cycle makes production costs competitive with the lower-cost methods of sailboard production outlined above.
An illustration of the low production costs using the method is provided by the U.S. Pat. No. 4,713,032 to Frank, the specification of which is incorporated herein, in which the prior art foamed epoxy laminate—often referred to as a “thermal compression-set epoxy” due to the high pressure and temperature cure—is replaced with a quickly-setting, foamed polyurethane resin for a very rapid mold-cycle of about twenty minutes per board, and high production from the molding tool of as many as twenty-four boards per day.
Using either resin, the gap between the interior core and the closed mold results in a fairly thick, resin-rich laminate, made weak by the foaming of the plastic in the skin. Because much higher-strength may be had using non-foaming structural layers of composite laminate, the strength-to-weight ratio of the foamed laminate skin is typically well below expensive high-performance sailboards, which eliminate the blowing agent in the resin matrix to create a much higher strength “structural sandwich” or “cored composite” skin.
b. Structural Sandwich/“Advanced Composite” Production
The structural sandwich is expensive to fabricate because of the lengthy mold-cycle. In production, vacuum pressure is used to remove voids and entrapped air from the composite laminate, and conforms the skin core to the shape of the mold; to prevent any spring-back of the skin core the skin material remains in the mold under vacuum pressure for about two to three hours, until the resin has completely cured. The added drawback is the difficulty removing excess resin from the skin—in sandwich skin fabrication, the skin core layer creates a buffer that blunts the effectiveness of the squeegee on the interior layers of laminate against the surface of the mold, and the mold's sharp, upraised edge contours tend to create a dam; the vacuum-bagging procedure then requires an airtight surface and seals the entire mold, and prevents any excess resin from escaping during cure. The further problem is that the stiffness of the core material generally exceeds the pressure available with vacuum (14.7 psi), which can prevent it from being fully conformed to the sharp curvature at the board's perimeter rail, particularly at the tail; this complicates the design of the joint and the overall structural integrity of the board as well.
The U.S. Pat. No. 4,964,825 to Paccoret, et al. illustrates a number of the problems outlined above; it reveals a large gap in the sandwich structure and very poor joint design at the board's perimeter edge (a conventional inward-turning bonding flange is depicted). During production, the mold's sharp edge contours, the fabrication of the sharply inward curving bonding flange, and the complete seal of the vacuum-bag combine to prevent any of the excess resin from escaping prior to cure. The invention is directed to structural improvements in the fin-box/mast track areas of the board; the design of the mold-seam and the removal of excess resin are much larger problems, but neither is addressed.
The U.S. Pat. No. 5,023,042 to Efferding discloses a mold designed specifically for sandwich skin construction; the complications in this case are due to the difficulties of using a low-density, pre-molded EPS (expanded polystyrene) “bead” foam for the interior core. In the disclosure, the PVC sheet foam/wet epoxy laminate fits into molded-in recesses in the EPS foam core and the entire assembly is placed in the mold, the exterior surface of which precludes resin removal by hand. Vacuum pressure is used to press the components tightly together but exceeds the compressive strength of the foam, causing it to distort and crush; the vacuum also withdraws air trapped between the individual beads of foam—the “outgassing” of air from the foam causes major structural defects in the form of large voids and pockets of entrapped air in the composite laminate.
To prevent these problems, Efferding suggests using a vacuum bore to withdraw the air from within the foam and discloses a novel mold with a flexible perimeter portion that, under full vacuum, bulges outward evenly and allows the EPS interior core to assume an even, permanent compression set during cure. Structural compromises include the large gap in the sandwich skin structure at the board's perimeter rail, and the absence of internal spars, shear webs, or hollow, weight-reducing areas in the board's interior core—all of which would create distortion problems and/or prevent the board from compressing evenly during cure. The added problem is the high resin content in the skin—to draw vacuum the mold completely encases and seals the board structure and prevents the excess resin applied in the laminating step from escaping during cure. A low temperature oven is also used to speed production, but costs are still very high due to the lengthy mold-cycle of just under three hours.
The inventor notes that known methods of sailboard production produce boards having a mold-seam at the point of greatest breadth thereof; the word “seamless” in the title of the invention refers to the modest improvement in the placement of the mold-seam—which is not in the expected location, but on the sharpest point of the rail.
Borrowing directly from the general mold configuration disclosed by Efferding, the U.S. Pat. No. 5,266,249 to Grimes III, et al., the specification of which is incorporated herein, teaches a method of forming interior joints in at least partially enclosed confined interior areas (see, e.g., the mold configuration to Smith) and an improved joint design as well, since the composite layers meet and form an overlapping joint at the perimeter rail. The fabrication of the joint, however, requires the use of extremely costly “advanced composite” material to prevent production problems caused by the mold's concave surface: the method uses the tackiness of the partially cured “pre-preg” epoxy laminate to adhere the deck layers of the honeycomb core to the walls of the mold; the bottom layers of the board are then assembled on an inflated bag, which doubles as a vacuum bag and provides the high outward pressure (i.e., at least 13 psi) needed to hold the composite skin/honeycomb core material in proper orientation and in pressurized contact with the mold throughout the cure.
According to the invention, there had previously been no method of applying fiber-reinforced plastic to the interior sides of mold-seams or joints; the inflated bag provides a very ineffective means for doing so, however, since its assembly adds considerable labor and limits the interior structure to a single support wall, rather than the higher strength and lighter weight afforded by a plurality of internal shear webs or supporting struts. Further, during assembly the inflated bag does not provide sufficient stability for the honeycomb core material to be accurately trimmed and glued—epoxy pre-preg core splice strips (strips of thermosetting epoxy that foam during the high-temp cure) are therefore required to fill the gaps or voids around the perimeter rail where the honeycomb core cannot be accurately fit.
Despite the obvious structural improvement over the prior art, the placement of the joint is still at the sharpest point on the board's perimeter rail, and its design is less than ideal—although the fabric is overlapped, the break in the reinforcing fiber and in the honeycomb core material will reduce the joint's shear and impact strength, and cause earlier skin detachment and/or failure of the joint at lower levels of impact. Much greater impact resistance can be had by eliminating the joint entirely: complete continuity of the core material would allow the continuity of the high-strength reinforcing fiber to be better maintained throughout the perimeter edge, and reinforcement could also be confined to the interior side. Higher impact strength could also be had by increasing the density of the core material throughout the exposed perimeter rail area itself.
More importantly, the mold's concave configuration makes it impossible to move the primary division between the two halves to the axis of symmetry—combining the joint with the support wall would create a far stronger board structure, since the support wall would provide an entire backup structure to reinforce the interior of the mold-seam in an area that is flat and only rarely exposed to high-point and impact loads.
As in the invention to Efferding, the mold completely encases and seals the board structure (bolts are depicted) and prevents any excess resin from escaping during cure. As noted above, an optimum fiber/resin ratio in the composite is extremely important: because the strength of the reinforcing fiber is usually several orders of magnitude higher than the resin (e.g., in a fiberglass composite, the tensile strength of glass, at roughly 500,00 psi, is about fifty times that of the resin, at 9-12,000 psi), excess resin in the composite actually weakens it. In sandwich skin fabrication, reducing the percentage of resin from the sixty to seventy percent range (by weight, and typical when the reinforcing fabric is saturated by hand) to the thirty-five percent level will usually double the compressive and flexural strength of the composite facing in bending—equally important, the weight saved can be used to increase the density of the stiff, lightweight sandwiched core. Because the improvement in strength that comes by increasing the density of the core is not linear—e.g., doubling the density of high-density plastic foam will usually triple its compressive strength—an optimum fiber/resin ratio in the laminate can more than double the flexural, compressive and impact strength of the structural sandwich skin as a whole.
Reducing the resin content of the laminate, however, requires special unidirectional fabrics or very tightly woven, difficult to saturate “crow-foot” or “satin” weaves of cloth, as well as a method for applying fairly high pressure to physically force the resin from the fiber, a lack of obstructions to allow the resin to actually be removed, and a barrier (e.g., a thin plastic film) to prevent air from re-entering the laminate (due to the slight spring-back of the fiber) once the pressure has passed. With current methods of production, efforts to employ these techniques have been less than completely successful because of the concave configuration of prior art molds, and the stiffness of the core material and the shape of the board at the rail—Paccoret et al. and Grimes III et al. therefore teach the use of extremely expensive “pre-preg” or “advanced composite” material to keep the resin content to an absolute minimum.
As the name suggests, in the “pre-preg” the reinforcing fabric is pre-impregnated with the precise amount of epoxy resin (by Hexcel, Ciba-Geigy etc.), the resin is then “B-staged” or partially cured, the material is shipped under refrigeration to the end-user (usually large airframe manufacturers such as Boeing etc.), the material is then placed in the mold and undergoes a high-temperature, high-pressure autoclave cure. Due to the prohibitively high material cost, the lengthy two to three hour mold-cycle (hr. to heat, 1-1 hr. cure, hr. to cool) and the high-temp, pressurized cure, the “advanced composite” or pre-preg laminate/honeycomb skin boards occupy only a small niche in the overall sailboard market.
The further drawback is that the generally hollow board structure is best used on very thick sailboards, where the higher overall volume of the interior foam core adds a great deal of weight but little strength, while a foam core may offer lighter weight with thinner, high-performance wave-boards (sailboards) and surfboards. The molds and methods outlined above, however, are not readily interchanged: the high-temperature and pressurized autoclave cure needed for the honeycomb core/pre-preg laminate (e.g., 250° F. and a minimum pressure of 13 psi), for example, typically exceeds the compressive strength of very low-density foams, and will melt polystyrene based foams (specified by Efferding, for example); special molds are also required for the mechanical/hydraulic press involved in attaining adequate adhesion using liquid resin pre-foams, whether the material is used as a fiber-reinforced foamed laminate in the skin or as the board's interior foam core. Hence, in the prior art, the configuration of the mold and the materials used in construction cannot be readily customized according to the design requirements of the board, or the performance preferences of the rider; compounding the problem, the mold's concave surface defines the board's exterior shape and restricts production to a series of exact duplicates.
Because of the very light weight and lower capital costs when the board is fabricated by hand, molded surfboard production has been very limited since the beginning of the “modern era,” which began with the introduction of moldable plastic foam and fiberglass-reinforced plastic over four and five decades ago respectively, while custom “one-off” sailboards comprise a very significant portion of the overall market—particularly in high performance areas such as Hawaii. In surfboard and high-performance sailboard production, the wide range in size and shape requires a large and prohibitively expensive inventory of molds, and eliminates the many custom design modifications that are now made as a matter of routine—the concave configuration of prior art female molds prevents the board's width, planing area and lengthwise bottom curvature or “rocker” from being tailored to the individualized requirements of the rider; custom boards, therefore, must be fabricated by hand.
c. Custom or “One-Off” Board Production
In custom or “one-off” surfboard production, the board is individually hand-shaped from a polyurethane foam “blank;” the fiberglass and resin are then applied by hand over the shaped foam core. The process is labor-intensive and requires considerable skill, but the problems of molded manufacture are limited to a pre-production phase—the board's interior foam core is first molded by a separate manufacturer into a rough surfboard-shaped slab of foam before being shipped to the surfboard manufacturer to be used in the actual construction.
To enhance strength and better control the somewhat unreliable reaction of the low-density polyurethane foam, the blank is molded in an extremely strong, heavy mold made of reinforced concrete. This allows an excess of liquid pre-foam to be poured in the mold; as the foam expands, the excess compresses under high pressure against the surface of the mold and produces a density-gradient in the blank—the foam is soft and weak in the center and becomes progressively harder and denser towards the surface. To avoid removing too much of the harder, denser surface foam during shaping, the blank is molded close-to-shape, or as thin as possible. The close-to-shape molding increases the large number of blank molds required for surfboard production, and frequently leaves insufficient foam in the nose and tail areas of the blank for the shaper to produce the proper lengthwise bottom curvature or “rocker” on the board.
The molded-in rocker of the blank must therefore modified by the blank manufacturer—this is done by cutting the blank in half lengthwise and gluing the two halves to a wooden center spar or “stringer” individually cut to a specific rocker curvature. The rocker is usually selected from a list of stock lengthwise rocker modifications; Clark Foam of Laguna Niguel, Calif., (www.clarkfoam.com) provides a Rocker Catalog listing the dimensions of over two thousand different templates available to modify the molded-in rocker curvature of the more than sixty different blank molds offered for surfboard production. With shipping and inventory problems at both ends of production, manufacture of the blank is expensive, but essential, since the hand-shaping of the core allows the various design parameters of the board, including the width, volume, and rocker, to be adjusted according to the requirements of the rider.
After shaping, the fiberglass laminate is applied directly to the shaped foam core, which provides a smoothly curving convex surface. With a fiber-reinforced composite, a convex substrate provides the foundation for a stronger, lighter structure—excess resin is easily removed for higher strength and lighter weight, and joint creation is stronger and simplified—the fabric can be pulled taut and a double overlapping joint created to provide a protective covering for the very exposed perimeter edge or “rail,” and the sharp convex curvature at the nose and tail as well.
Major drawbacks include the large amount of labor and extra coats of resin required to sand the overlapped area completely smooth; structurally, the primary problem is the board's very light weight—for higher performance, board weight has been consistently reduced to the point where the low-density interior foam core is no longer strong enough to fully support the board's thin exterior skin. The single fiberglass ply used on the bottom of the board will usually dent or fracture with moderate finger/thumbnail pressure, while the double or triple layer used to reinforce the tail area where the rider stands often fatigues, becomes permeable to water, then fails and completely delaminates under the repeated high pressure of the rider turning the board. Hand-shaping also limits the effectiveness of the longitudinal reinforcement—it makes wood the material of choice for the center spar and also makes it impractical to add top and bottom spar caps (i.e. the top and bottom reinforcing flanges in an I-beam)—the lack of effective longitudinal reinforcement leaves thinner surfboards in particular susceptible to breakage.
With current methods of production, the strength of the “one-off” or custom board is severely compromised by the roughly one-to-one weight ratio between the fiberglass skin and interior foam core—efforts to alter this ratio have been largely unsuccessful. The U.S. Pat. No. 5,569,420 to Van Horne, for example, suggests increasing the density of the polyurethane foam core—this is done by pouring out sequential lines of liquid pre-foam; the foam then expands, and each individual line of foam is left with a hardened arcuate shell on its exterior surface where the exposure to air has slowed the reaction of the foam. The process is repeated until a billet is formed; the foam for the board's interior foam core is then cut from the billet and hand-shaped the final dimensions. Although the density of the foam is increased, the invention eliminates one of the primary advantages of the one-off method of production, which is the extremely rapid mold-cycle in the molding of the blank (e.g., in the invention to Frank, a reaction retarder is needed to extend the rapid five minute setting time of the polyurethane resin in the foamed, fiber-reinforced plastic skin).
The U.S. Pat. No. 4,255,221 to Young teaches a laminated plywood skin created from individual layers of veneer which are conformed to the curvature of a hand-shaped interior foam core using vacuum pressure. To reduce weight, Young provides additional adjustable means outside the vacuum forming apparatus that squeeze excess epoxy from the layers and aid in conforming the wood to the curvature of the core. The difficulty is in forming an effective joint at the perimeter rail—since the veneer can break if the curvature is severe, the edge contours of the board are made by laminating strips of wood around the board's perimeter; after curing, the strips are hand-planed to the final dimensions and form a solid laminated wood perimeter rail.
In the disclosure to Efferding discussed above, the inventor describes developing a similar vacuum-bagging method for fabricating a sandwich skin sailboard. The use of composite skin core material greatly complicates the process, however. For example, at room temperature PVC sheet foam will typically break well before it reaches a right-angled bend, and its stiffness can exceed the 14.7 psi pressure available with vacuum and the compressive strength of the low-density EPS foam core as well; in addition, the lack of any reference point around the board's perimeter edge makes it very difficult to accurately trim the skin core material and eliminate the potential gap or mismatch between the board's two opposing sides. The inventor therefore makes no attempt to conform the high-density PVC sheet foam to the sharp curvature at the perimeter rail, but teaches that the skin core should be fit into recesses in the foam core. Efferding reports that it takes thirty to forty five hours to manufacture a professionally acceptable sailboard using the technique; to reduce labor, Efferding discloses and teaches the use of a novel female mold.
Prior art custom or “one-off” production currently lacks a strong, composite based skin structure capable of being fabricated on a convex surface such as plastic foam; in molded manufacture, the high production costs and structural problems are largely due to the concave configuration of prior art female molds and the placement of the part-line between the mold's top and bottom halve, which results in a mold-seam along the board's perimeter edge or rail. In the prior art, the two basic fabrication methods fail to address specific problems encountered in conforming the exterior skin materials to the sharp compound curvature at the board's perimeter rail, and suffer from a number of serious structural shortcomings and manufacturing drawbacks as a result.